As a semiconductor device in which pixels each provided with a photosensor are arranged in a matrix, a complementary metal oxide semiconductor (CMOS) image sensor is known. CMOS image sensors are provided in many portable devices such as digital cameras or cellular phones as imaging elements.
Silicon is widely known as a semiconductor material applicable to a transistor generally included in a CMOS image sensor or the like. As another material, an oxide semiconductor has attracted attention.
For example, Patent Document 1 discloses that a transistor including an oxide semiconductor and having extremely low off-state current is used in part of a pixel circuit and a transistor including a silicon semiconductor with which a CMOS circuit can be formed is used in a peripheral circuit, so that an imaging device with high speed operation and low power consumption can be manufactured.